This invention relates to a bus interface system for coupling memory to processing circuits in a data processing system such as a calculator. More particularly, this invention relates to a common bus for address, data, and command (control) signals without requiring a separate bus for identifying whether address, data, or control signals are presently being transferred on the common bus as heretofore required. Alternatively, separate buses have been utilized for conducting data, control, and address signals between said memory circuits and said data processing circuits. However, the use of separate buses, either for identifying the type of signal on common bus, or for each carrying a separate type of signal requires additional circuitry and increases the physical size of the data processing system utilizing such buses.